Compact disk (CD) recordings are a preferred means for distributing both digital audio recordings and computer software. CD digital audio (CD-DA) recordings are almost universally made in accordance with standard 908 of the International Electrotechnical Commission (IEC), entitled “Compact Disc Digital Audio System” (Geneva, Switzerland, 1987), which is incorporated herein by reference. The 908 standard, also known as the Red Book, defines how audio data are to be encoded and recorded on the disk, enabling the data to be played back using standard digital decoding chips.
In accordance with the Red Book standard, the data on the CD are recorded in blocks (or sectors) of 98 frames, each such frame comprising one control symbol followed by thirty-two data symbols. Each symbol comprises fourteen channel bits, which correspond to a single byte of binary input data, encoded in a process of eight-to-fourteen bit modulation (known as “EFM” modulation). The data symbols include error correction codes, known as Cross Interleave Reed Solomon Codes (CIRC), which are used during playback to identify and, when possible, to correct errors in the corresponding data segments. The control symbols contain sub-code information for the block, arranged in channels that are identified in the standard as P, Q, R, S, T, U, V and W. The first two sub-code symbols contain synchronization patterns known as S0 and S1. The blocks are organized sequentially in tracks, beginning with a lead-in track 0, followed by up to 99 audio tracks. The P and Q sub-code channels contain timing and navigation information for the tracks. The Q channel in the lead-in contains the Table Of Contents (TOC) of the disk.
Different standards apply to other types of data, such as computer software, which may be recorded on CD, as well. For example, the Blue Book standard specifies multi-session recordings, that combine audio and other data on a single disk. The Orange Book applies generally to recordable CDs (CD-R), including rewritable CDs (CD-RW), which allow users to write, erase, overwrite and read CD information using properly-configured recording and playback equipment. The CD-RW format permits both audio and data recording. It is described in detail in part II of the Orange Book, which is entitled “Compact Disc ReWritable System Description” (Tentative Version 1.90, June, 1997), published by Philips Consumer Electronics B. V. (Eindhoven, Netherlands), which is incorporated herein by reference.
Inexpensive CD recording devices and read/write media that comply with Orange Book requirements have become available to consumers. Thus, it is now easy, using a conventional personal computer with a CD burner, to make exact, digital copies of all types of CDs. The CD content is read out by the computer, byte by byte, from one CD, typically a legitimately-purchased original CD, and then written to a blank CD-R or CD-RW so as to make a pirate copy. Similar copying methods may be used with other media, such as Digital Video Disks (DVD). Off-shelf software to assist the user in making such copies is also widely available. For example, Roxio™ Inc. (Milpitas, Calif.) offers “Easy CD Creator” software, which is commonly bundled with CD burners sold on the consumer market. Such software typically contains no program safeguards to prevent users from making unauthorized copies of copyright material. Financial losses to the recording and software industries due to such pirate copying are estimated to be in the billions of dollars.
Various methods are known in the art for protecting CD audio content from being copied using computer CD drives. Some of these methods are based on manipulating the sub-code control information. For example, PCT patent publication WO 01/61696, whose disclosure is incorporated herein by reference, describes a method for copy protection of digital audio CDs based on using a non-standard number of frames in a sector of a compact audio disk, for example, 99 rather than the usual 98 frames specified by the standard. The S0 and S1 sub-code synchronization patterns are therefore displaced one frame later than they would normally be, without changing the data in the frames. As another example, PCT patent publication WO 00/74053, whose disclosure is likewise incorporated herein by reference, describes a copy protection method based on rendering certain of the sub-code information, particularly in the Q channel, incorrect and/or inaccurate. The incorrect data is ignored by an audio CD player, but renders the disk unplayable by a computer CD drive.
Other methods of copy protection involve changing the audio content of the original disk. For example, PCT patent publication WO 01/41130, whose disclosure is incorporated herein by reference, describes such a method based on incorporating a pattern of latent noise into a digital audio CD by overwriting some of the original audio signal data symbols with grossly-erroneous values. The corresponding error-correction symbols are overwritten in such a way as to create an uncorrectable error in the frames containing the erroneous values. An ordinary CD player, when playing such a disk, will detect each occurrence of a latent noise value as an uncorrectable error and will apply interpolative error-concealment to prevent the output of the error. When a computer CD drive attempts to read the disk, however, and detects the intentional, uncorrectable errors, it will either read no data at all or read only the raw uncorrected data, including the latent noise.
One way in which CD-RW disks used to produce unauthorized copies of copyright material differ from CD-DA and other types of non-rewritable disks is that they typically contain a Program Memory Area (PMA), preceding the conventional Lead-In Area of the disk. The PMA is used for intermediate storage of control information during the process of recording the disk. This information includes:                Disk contents—track numbers and their start and stop times.        Disk identification.        Skip information—indicating audio tracks or parts of audio tracks to skip during playback.When the entire disk is finalized, the information in the PMA is encoded in the sub-code Q channel of the lead-in, and the PMA is erased. It is not necessary that the disk be finalized, however, in order for its contents to be played back. Most currently-available CD drives are designed for CD-RW compatibility and therefore are programmed to read and use any PMA information on the disk before going on to the lead-in and program areas of the disk. Only in Disk-At-Once (DAO) CD-RW recordings, in which the entire disk contents are copied in one operation, is the sub-code Q channel information written to the disk directly, without going through the PMA. Thus, DAO recordings, such as those made in manufacturing plants, conventionally have no PMA. CD-DA players ignore the PMA in any event.        